Scientists have devised alternative ways to study ocean food webs using stable isotope analysis – particularly for carbon and nitrogen. Isotopes represent variants of an element that differ only in the number of neutrons present, just slightly altering the mass of a molecule that incorporates the different isotopes. By following the changes in ratios of different isotopes (noted as δ), scientists can glean all sorts of information about food web interactions. Carbon and nitrogen isotope signals (δ13C and δ15N) ­are useful since they get altered in a relatively systematic way through each trophic level, giving scientists information as to the exact trophic level an organism occupies (Figure 1).

Scientists can get even more information by looking at the isotopic signal in specific compounds, such as amino acids. Essential and source amino acids are compounds that certain animals are unable to synthesize themselves but are required for important protein synthesis and therefore must be obtained through food. Since these amino acids are directly incorporated into an animals’ biomass from their food, these amino acid isotopic signatures remain relatively unchanged, thus reflect the isotopic composition of their food source. In contrast, non-essential or trophic amino acids can have altered isotopic signatures which are more informative about things such as trophic level.

Dr. Rocio Ruiz-Cooley and colleagues from the University of California, Santa Cruz recently employed amino acid stable isotope analysis to study the changes occurring in the eastern portion of the California Current system. They used a long time series to characterize any changes that have occurred in the system over recent time.

Methods

Skin biopsy samples were collected from individual sperm whales. Seventeen samples were taken in total, either from live individuals off the California Coast system or from beached individuals from California up to Washington (Figure 2).

Figure 2. Skin biopsy samples were taken from 17 individual sperm whales from 1993-2005. Black circles are tissue samples used for bulk stable isotopes and red circles are samples used for amino acid stable isotope analysis.

Tissues were analyzed for bulk isotopic composition by using an isotopic ratio mass spectrometer (IRMS), which is able to separate and quantify compounds based on different isotopic ratios. Four source amino acids (phenylalanine, glycine, lysine, and tyrosine) and five trophic amino acids (glutamic acid, alanine, isoleucine, leucine, and proline) were extracted from tissue samples and run through IRMS to determine the nitrogen isotopic composition of amino acids. Essential (phenylalanine, valine, and leucine) and non-essential (alanine, proline, aspartic acid, glutamic acid, and tyrosine) amino acids were extracted and run through the IRMS to determine the carbon isotopic composition of the amino acids. A sample from 1972 was also included in bulk and amino acid isotope analyses to potentially give an even longer study period to observe.

Results:

Dr. Ruiz-Cooley and colleagues found a consistent and statistically significant decline of both bulk and amino acid for δ15N and δ13C (Figure 3). From 1993 to 2005, δ15N decreased by 1.7 0/00. Including the sample from the 1972 tissue samples show a decrease of greater than 3 0/00 between 1972 and 2005. δ13C decreased by 1.1 0/00 between 1993 and 2005 and decreased by more than 4 0/00 from 1972 to 2005.

Figure 3. Bulk and amino acid specific δ15N (top) and δ13C show a significant decrease over the sampling period.

Scientists have observed changes in bulk δ15N and δ13C values in other systems in response to changes in system productivity. Large shifts in bulk δ15N can represent changes in system productivity, where increased primary production might allow higher trophic level organisms to shift down the food web (allowing them to use multiple trophic levels when conditions are good and food is plentiful). The opposite can also happen – decreases in primary productivity may force organisms to consume at higher trophic levels to sustain their baseline nutrient and energy requirements. However, the declines in bulk δ15N and δ13C observed by Dr. Ruiz-Cooley and colleagues was not a large enough shift to suggest a change in trophic level. Thus, something else must be occurring.

An alternative explanation for changing bulk δ15N and δ13C could be a system-wide shift in isotopic composition, from the phytoplankton up to whales. This is where amino acid stable isotope analysis comes into play. Source and essential amino acids (which track nitrogen and carbon isotopes, respectively) undergo little to no modification as they pass through the food web and remain more or less unaltered, even in very high trophic organisms such as whales. Dr. Ruiz-Cooley and colleagues observed a similar, parallel trend in their trophic and non-essential amino acids as in their source and essential amino acids. Thus, changes in bulk δ15N and δ13C observed in the whales can be explained by changes in the overall system baselines.

Implications:

While not readily apparent, this decrease in bulk δ15N and δ13C indicate a changing system while the magnitude of decrease and parallel trends in lower trophic level proxies show that overall ecosystem structure is preserved. Dr. Ruiz-Cooley hypothesize that climate variability (such as Pacific Decadal Oscillation – Figure 4) or climate change effects may cause widespread changes in ocean biogeochemical cycling.

Figure 4. Time series of sea surface temperature in the California Current System (inset) showing the influence of Pacific Decadal Oscillation. The red line shows a regression line of sea surface temperature during the period of this study, showing a general cooling pattern which may cause changes in the system that might contrast with predicted climate change effects.

For example, warming may increase stratification and decrease nutrient exchange, which would then decrease the available nutrient for primary production. This might shift plankton communities to favor nitrogen fixers (like cyanobacterium) which can create their own nutrients from atmospheric nitrogen. This shift in biogeochemical process would shift the isotopic signal of the phytoplankton, and this signal change would continue throughout the rest of the food web. However, climate change and climate variability research shows conflicting or inconclusive results, making it hard to make guesses as to what is driving these changes, let alone develop a hypothesis to move forward.

A recent convert to oceanography, I’m studying under Dr. Anne McElroy at Stony Brook University’s School of Marine and Atmospheric Sciences. My research uses biochemical and genomic methods to investigate how coastal organisms respond to environmental stress.